target-mips: add PC, XNP reg numbers to RDHWR

Add Performance Counter (4) and XNP (5) register numbers to RDHWR.
Add check_hwrena() to simplify access control checkings.
Add RDHWR support to microMIPS R6.

Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
diff --git a/target-mips/cpu.h b/target-mips/cpu.h
index c68681d..fa919c1 100644
--- a/target-mips/cpu.h
+++ b/target-mips/cpu.h
@@ -469,6 +469,7 @@
 #define CP0C5_CV         29
 #define CP0C5_EVA        28
 #define CP0C5_MSAEn      27
+#define CP0C5_XNP        13
 #define CP0C5_UFE        9
 #define CP0C5_FRE        8
 #define CP0C5_SBRI       6